Drawbridge

A bridge with a movable section that can be raised or swung aside to let ships pass or block access. Its mechanism and purpose vary between waterways and fortified entrances.

Bascule bridge: A bridge whose span rotates upward around a horizontal pivot, often counterweighted. Its rotating leaf is a common drawbridge form for opening navigable channels.
Medieval fortification: A defensive structure or system built in Europe during the Middle Ages. Fortified entrances made movable bridges useful for controlling access.
Chicago River: A river flowing through Chicago whose branches connect to the Great Lakes and Illinois waterways. Its many movable bridges accommodate vessels within a dense city street network.
Fixed bridge: A bridge whose span remains stationary and does not move to clear traffic. It cannot open for vessels, so its deck must provide sufficient clearance or restrict passage.
Counterweight: A mass used to balance or offset the load of a moving structure. Counterweights reduce the force needed to raise many bascule spans.
Castle moat: A defensive ditch, often water-filled, surrounding a castle or other fortification. A drawbridge could cross the moat and be raised to obstruct entry.
Tower Bridge: A combined bascule and suspension bridge spanning the River Thames in London. Its twin bascules lift to let tall vessels pass through central London.
Vertical-lift bridge: A bridge whose deck rises vertically between towers while remaining horizontal. Unlike a rotating drawbridge leaf, its span lifts straight upward.
Bridge operator: A person who controls a movable bridge and coordinates its opening and closing. Operators time openings with vessel traffic and keep roadway users clear.
Portcullis: A heavy barred gate that moves vertically in grooves at a fortified entrance. It often complemented a drawbridge as another barrier at castle entrances.

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